STOMATA. 



85 



there rose-coloured. The long axis of the stoma corresponds 

 with the long axis of the leaf, so that here also it is easy to 

 obtain correct cross-sections. The stomata present then the 

 appearance shown in Fig. 30, B. The side of the guard-cells 

 towards the cleft appears in this case also to be less thickened at 

 about rnid height, while the whole of each side turned towards 

 the adjoining epidermal cells is also thinner. Besides this, it 

 happens that both of the epidermal cells adjoining the guard-cells 

 are flatter than the epidermal cells lying beyond, and are also 

 less thickened on their outer sides. They appertain, therefore, 

 to the stornatic apparatus as "accessory cells," and stand in 

 lieu of the hinge or joint which in Iris florentina is formed 

 merely by the thin part of the membrane at the junction with 



FIG. 30. Epidermis of the under side of the leaf of Tradescantia virginica,. A t 

 seen from above ; B, in cross-section through the leaf ; /, leucoplasts upon the nucleus 

 ( x 240). 



the guard-cells. The leucoplasts (/), which surround the nucleus 

 in the epidermal cells offer here a very favourable object for 

 observation. It is interesting that these leucoplasts, in spite of 

 being in a position so strongly exposed to the light, remain small 

 and colourless, and do not develop into chlorophyll grains ; but 

 this is connected with the fact that the epidermis has here no 

 assimilating function. 



Epidermis of T. zebrina. Tradescantia zebrina has a stomatic 

 apparatus composed in the same way, and can replace T. virginica 

 when the latter is not available. Stomata are present only on 

 the under side of the leaf. Many epidermal cells contain red 

 cell-sap. On each side of the stomatic apparatus not one but 

 two accessory guard-cells are usually present. The cross-section 

 is very instructive, though not easy to obtain sufficiently thin ; 



